Polysaccharides are polymers that include monosaccharide (sugar) units connected to each other via glycosidic bonds. These polymers have a structure that can be described in terms of the linear sequence of the monosaccharide subunits, which is known as the two-dimensional structure of the polysaccharide. Polysaccharides can also be described in terms of the structures formed in space by their component monosaccharide subunits.
A chain of monosaccharides that form a polysaccharide has two dissimilar ends. One end contains an aldehyde group and is known as the reducing end. The other end is known as the non-reducing end. A polysaccharide chain may also be connected to any of the C1, C2, C3, C4, or C6 atom if the sugar unit it is connected to is a hexose. In addition, a given monosaccharide may be linked to more than two different monosaccharides. Moreover, the connection to the C1 atom may be in either the α or β configuration. Thus, both the two-dimensional and three-dimensional structure of the carbohydrate polymer can be highly complex.
The structural determination of polysaccharides is of fundamental importance for the development of glycobiology. Research in glycobiology relates to subjects as diverse as the identification and characterization of antibiotic agents that affect bacterial cell wall synthesis, blood glycans, growth factor and cell surface receptor structures involved in viral disease, and autoimmune diseases such as insulin dependent diabetes, rheumatoid arthritis, and abnormal cell growth, such as that which occurs in cancer.
Polysaccharides have also been used in the development of biomaterials for contact lenses, artificial skin, and prosthetic devices. Furthermore, polysaccharides are used in a number of non-medical fields, such as the paper industry. Additionally, of course, the food and drug industry uses large amounts of various polysaccharides and oligosaccharides.
In all of the above fields, there is a need for improved saccharide analysis technologies. Saccharide analysis information is useful in, e.g., for quality control, structure determination in research, and for conducting structure-function analyses.
The structural complexity of polysaccharides has hindered their analysis. For example, saccharides are believed to be synthesized in a template-independent mechanism. In the absence of structural information, the researcher must therefore assume that the building units are selected from any of the saccharide units known today. In addition, these units may have been modified, during synthesis, e.g., by the addition of sulfate groups.
Second, saccharide can be connected at any of the carbon moieties, e.g., a the C1, C2, C3, C4, or C6 atom if the sugar unit it is connected to is a hexose. Moreover, the connection to the C1 atom may be in either α or β configuration.
Third, saccharides may be branched, which further complicates their structure and the number of possible structures that have an identical number and kind of sugar units.
A fourth difficulty is presented by the fact that the difference in structure between many sugars is minute, as a sugar unit may differ from another merely by the position of the hydroxyl groups (epimers).
The use of a plurality of such saccharide-binding agents, whether fixed to the substrate and/or employed as the second (soluble) saccbaride-binding agent, characterizes the carbohydrate polymer of interest by providing a “fingerprint” of the saccharide. Such a fingerprint can then be analyzed in order to obtain more information about the carbohydrate polymer. Unfortunately, the process of characterization and interpretation of the data for carbohydrate polymer fingerprints is far more complex than for other biological polymers, such as DNA for example. Unlike binding DNA probes to a sample of DNA for the purpose of characterization, the carbohydrate polymer fingerprint is not necessarily a direct indication of the components of the carbohydrate polymer itself. DNA probe binding provides relatively direct information about the sequence of the DNA sample itself, since under the proper conditions, recognition and binding of a probe to DNA is a fairly straightforward rocess. Thus, a DNA “fingerprint” which is obtained from probe binding can yield direct information about the actual sequence of DNA in the sample.
By contrast, binding of agents to carbohydrate polymers is not nearly so traightforward. As previously described, even the two-dimensional structure (sequence) of carbohydrate polymers is more complex than that of DNA, since carbohydrate polymers can be branched. These branches clearly affect the three-dimensional structure of the polymer, and hence the structure of the recognition site for the binding agent. In addition, recognition of binding epitopes on carbohydrate polymers by the binding agents may be affected by the “neighborhood” of the portion of the molecule which is surrounding the epitope. Thus, the analysis of such “fingerprint” data for the binding of agents to the carbohydrate polymer of interest is clearly more difficult than for DNA probe binding, for example.
A useful solution to this problem would enable the fingerprint data to be analyzed in order to characterize the carbohydrate polymer. Such an analysis would need to transform the raw data, obtained from the previously described process of incubating saccharide-binding agents with the carbohydrate polymer, into a fingerprint which would itself contain information. The fingerprint would also need to be standardized for comparison across different sets of experimental conditions and for different types of saccharide-binding agents. Unfortunately, such a solution is not currently available.
In spite of these difficulties, a number of methods for the structural analysis of saccharides have been developed. For example, PCT Application No. WO 93/24503 discloses a method wherein monosaccharide units are sequentially removed from the reducing end of an oligosaccharide by converting the monosaccharide at the reducing end to its keto- or aldehyde form, and then cleaving the glycosidic bond between the monosaccharide and the next monosaccharide in the oligosaccharide chain by using hydrazine. The free monosaccharides are separated from the oligosaccharide chain and identified by chromatographic methods. The process is then repeated until all monosaccharides have been cleaved.
PCT Application No. WO 93/22678 discloses a method of sequencing an unknown oligosaccharide by making assumptions upon the basic structure thereof, and then choosing from a number of sequencing tools (such as glycosidases) one which is predicted to give the highest amount of structural information. This method requires some basic information as to the oligosaccharide structure (usually the monosaccharide composition). The method also illustrates the fact that reactions with sequencing reagents are expensive and time-consuming, and therefore there is a need for a method that reduces these expenses.
PCT Application No. WO 93/22678 discloses a method for detecting molecules by probing a monolithic array of probes, such as oligodeoxynucleotides, immobilized on a VLSI chip. This publication teaches that a large number of probes can be bound to an immobilized surface, and the reaction thereof with an analyte detected by a variety of methods, using logic circuitry on the VLSI chip.
European Patent Application No. EP 421,972 discloses a method for sequencing oligosaccharides by labeling one end thereof, dividing the labeled oligosaccharide into aliquots, and treating each aliquot with a different reagent mix (e.g. of glycosidases), pooling the different reaction mixes, and then analyzing the reaction products, using chromatographic methods. This method is useful for N-linked glycans only, as they have a common structure at the point where the saccharide chain is linked to the protein. O-linked glycans are more varied, and the method has as yet not been adapted for such oligosaccharides with greater variability in their basic structure.
There is therefore a need for a system and method for characterizing polysaccharides using an accurate, high throughput method for identifying agents that bind to the polysaccharide.